The present invention relates to an apparatus for measuring the luminescence of a semiconductor sample spatially resolved.
To ensure clarity, it is necessary to establish the definition of several important terms and expressions that will be used throughout this disclosure.
The term “spatially resolved” means that the measured luminescence data can be assigned uniquely to singular points of semiconductor surface. If the measurements are based on a Cartesian coordinate system, for example, the luminescence data are sampled together with the x and y coordinates of the measurement points in reference to a point of origin on the semiconductor sample. Alternatively, if the measurements are based on a polar-type coordinate system, for example, the luminescence data is sampled in reference to the angle of a rotatable sample holder and in reference to the linear distance from a point of origin. The point of origin may be, for example, the centre point of the semiconductor sample. With the term “spatially resolved” it is also meant that the semiconductor sample or a part of it is scanned in a two-dimensional way for acquiring the measurement data.
The term “semiconductor sample” relates to a flat sample, which includes completely or in part one or several semiconducting materials, or which is coated at the surface area with at least one semiconducting layer. This sample can be either a flat planar sample without structures, or it can be a sample, which has been processed already by some so-called photolithographic steps, or it can also be a sample with metal contact layers. The surface of the sample can be further coated with translucent layers, for example, layers which are commonly used for passivation in semiconductor technology.
The term “rapidly” or “rapid rotation” relates to rotations in the region of 0.5 to 10 revolutions per second, preferably in the range of 1 to 5 revolutions per second.
To measure the luminescence of semiconductor samples, two measurement apparatus types are known, which hereinafter are referred to as type A and type B.
In a measurement apparatus of type A, the semiconductor sample is driven to a certain point using an xy stage, then stopped, and then a luminescence measurement is performed. For the luminescence measurement, for example, a focused laser beam is used to stimulate luminescence at the sample surface, and the luminescence light thus generated is investigated in a spectrometer. A corresponding apparatus is described in the publication “Publikation des Lehrstuhls für Moderne Optik und Quantenelektronik (LS Leitenstorfer)—Fachbereich Physik—Uni Konstanz” [Publication of the Chair for Modern Optics and Quantum Electronics (LS Leitenstorfer)—Faculty of Physics—University of Constance] (“http://www.uni-konstanz.de/quantum-electronics/index3.php?lg=en&sub=4&sub2=1”) together with FIG. 2. The laser light passes with its wavelength of 488 nm or 532 nm trough a semi-translucent mirror on a lens, which focuses the laser light on the surface of the sample. This same lens is used to collect the resulting luminescence light. The luminescence light has a smaller energy, i.e. higher wavelength than the stimulating laser light. The semi-translucent mirror is designed such, that it transmits the stimulating laser light, but the longer wavelength luminescence light is reflected, such that the luminescence light is mirrored into the spectrometer for analysis, and the stimulating laser light does not reach the spectrometer. Thus, the spectrometer can be operated at a very high sensitivity, and the stimulating laser light with high intensity will not disturb the measurement of the luminescence light. The laser light hits the semiconductor surface perpendicularly. It is important that the semiconductor sample is located in the focus area of the lens. The focus of the lens can be adjusted by varying its height. The sample and the xy-table are mounted in a low-temperature cryostat. The sample can be cooled down to Helium temperature (4.2K). Typically only relatively small pieces of a semiconductor sample are measured, which fit in the relatively small cryostat. It is not possible, to measure a complete semiconductor with a typical size of 2 to 12 inch diameter. The experimental setup is a laboratory setup and is not intended to be sold as a complete system or apparatus.
A measurement setup according to type A is well known also from Appl. Phys. A 40, p. 191-195 (1986), especially p. 191, right column, last paragraph, to page 192, left column, last paragraph. The measurement here is described as “PLtop” measurement. Here laser light with a wavelength of 514.5 nm is focussed to the surface of a wafer at an angle of about 45 degrees relative to the wafer surface. Stimulated luminescence radiation is collected, passed through 2 filters and then focused into a photomultiplier. It is worth noting here that here—controlled by a computer—a spatially resolved measurement is carried out, but only at room temperature (300K). The article does not specify the measurement time to map a complete semiconductor sample. But experience shows that the mapping of an entire semiconductor wafer with such a setup using an xy stage takes relatively long time.
Another measurement setup in accordance with type A is described in the U.S. Pat. No. 6,075,592. Here, again laser light is focussed vertically on a semiconductor sample. The diameter of the focal point is about 5 μm or 10 μm. The semiconductor sample is placed on a translation stage, so that it can be moved in x, y and z direction. Further there is the possibility to turn the sample initially, i.e. before the actual measurement, if necessary. The excited luminescence light is collected through the same lens and a dichroic coupler is used to separate it from the stimulating laser light, the luminescence light is then sent into an Optical Spectrum Analyzer for analysis. This publication provides no direct reference to the temperature at which is measured. From the details presented it is concluded that the measurements are done at room temperature.
It should be noted, that the turning movement here is meant in the sense of “displacement in turning direction”, but not in the sense of “relatively rapid rotation during the measurement”—as it is required for the apparatus described here-in. In other words, the turning of this sample holder serves only for a slow and precise alignment of the semiconductor sample. During the actual measurement the sample holder is not turned. Thus it may be aligned in rotational direction around the z axis; after this alignment the sample holder is not turned further and the actual measurement starts.
European Pat. No. EP 0 925 497 B1 describes an equipment of the type A, which uses a maximum “scan area” of 1 mm×1 mm, which is scanned in the range of minutes. According to its description, this patent is completely restricted to an equipment of the type A. The measurement is performed only at room temperature.
European Pat. No. EP 0 545 523 B1 describes also an equipment of the type A. Here again a fast rotation is not used. The laser used here hits the semiconductor surface under an inclined angle. The excited radiation is collected vertically from the surface and guided into a spectroscope.
In a setup of type B for spatially resolved measurements, the semiconductor sample is fixed on a sample holder which can be rotated horizontally. This sample holder rotates around its vertical axis, while it is shifted in one linear direction of the xy-plane at the same time. Such a design of type B, on which the invention is based as will become apparent later-on, is well known from Applied Physics A50, p. 531-540 (1990), FIG. 1, in conjunction with section 1.1 and 1.2. There the procedure is named PLT procedure (photoluminescence topography). The rotating wafer plate is shifted in this one direction until the entire semiconductor sample has passed a fixed measurement point (focal point) in form of a spiral. At this fixed measurement point the semiconductor sample is hit by a focused laser beam of the wavelength 325.0 nm, 441.6 nm or 632.8 nm. Using a special optical device to focus the laser beam, the laser beam has a radially elongated form of 30 μm*200 μm where it hits the semiconductor surface. Using an optical device which is mounted vertically above the fixed measurement point, the stimulated luminescence light is collected, the laser wavelength is filtered out, and the resulting light is fed into a detector which consists of a photomultiplier. According to this article, an entire semiconductor wafer can be scanned and measured completely.
The apparatus of type B is relatively fast, because the movement of the semiconductor wafer is not stopped during the measurement. But with the apparatus of type B, it is not possible to measure semiconductor samples at low temperatures down to the boiling point of Helium (4.2K) and below. To achieve the low temperatures here, the sample holder with semiconductor would be needed to be in very good thermal contact with liquefied gas. Typically, liquid nitrogen or liquid helium is used. This would require a permanent mechanical connection between the sample holder and a pipe to a liquid gas tank. It is practically impossible, to install this mechanical connection such that the sample holder can rotate. Further the liquid gas tank is so heavy and large, that it would not be practical to rotate it together with the sample holder at a fast speed.
Looking at the currently applied handling in semiconductor laboratories or production facilities, sometimes a device is used to measure relatively large semiconductor samples rapidly at room temperature with a rotatable sample holder, and in addition a device without rotation for low-temperature measurements of relatively small semiconductor samples is used. This approach has the drawback that the complexity is very high. Often such measurements are performed in a clean room, where space is very expensive and limited. In addition, the optical devices used to focus the excitation light, the excitation light sources (lasers), the optical devices to collect the luminescence light, and the spectrometers used as precision detectors are partly very complex, delicate and expensive optical-mechanical devices, which require complex adjustment and calibration steps. If two such devices are used, the complete efforts are typically doubled. Further in many cases it is desirable to compare wide area room temperature measurements with local measurements at low temperature. It is unfavorable to use two different optical systems and/or detectors for such a comparison.
It would therefore be desirable and advantageous to provide an improved measurement apparatus for spatially resolved measurements of luminescence light generated in and emitted from semiconductor samples to obviate prior art shortcomings.